Method of impregnating a porous electrical coil



April 26, 1955 J o FENWlCK 2,707,155

METHOD OF IMPREGNATING A POROUS ELECTRICAL COIL Filed April 27, 1951 INVEN TOR.

4052 af2ww HZZorne United States Patent METHOD OF IMPREGNATING A POROUS ELECTRICAL COIL John O. Fenwick, Kentland, Ind., assignor to McGraw Electric Company, Milwaukee, Wis., a corporation of Delaware Application April 27, 1951, Serial No. 223,263

9 Claims. (Cl. 117-49) This invention relates to a new and improved electrical induction apparatus and method of manufacturing said apparatus.

A major factor determining performance characteristics of oil-filled transformers has been the impregnation method used to protect the coils, or core and coil assemblies. Properly performed impregnation methods contribute to high surge and short-circuit strength. It has been proven that a transformer impregnated with oil has an inherent surge strength greater than units bonded with gum or resin. In small sizes where sufficient short-circuit strength can be maintained by the mechanical structure; i. e. clamps, brackets, etc., the plain oil impregnated transformer is superior. On larger transformers, it has been found desirable to continue some kind of bonding impregnant to increase the shortcircuit strength above that which could conveniently be provided by mechanical structures.

Reference is made to copending application, Serial No. 223,187, filed on April 27, 1951, by Martin I. Zwelling, and assigned to the same assignee as the present invention. The copending application relates to the present invention, and includes additional improvements pertaining thereto.

At one time, it was the general practice to use a natural gum for the impregnation of transformers. The only other type of impregnation used at that time was the process utilizing an oleo-resinous varnish. This type of varnish proved to be difficult to bake to a dry condition in the interior layers of the coil windings, as the baking operation produced a film on the outside of the coil which prevented air from reaching the interior layers. Oxygen in the air was also necessary for complete curing or drying of the oleo-resinous varnishesv The next stage in the development of an irnpregnant was the use of a synthetic heat-reactive varnish. This type of material does not require the presence of oxygen for curing or hardening. Heat is the only requirement. When this type of material is properly formulated, applied, and cured, it will provide a higher resistance to transformer oil than any other known kind of bonding impregnant. Varnishes of this class include, among others, oil-modified phenolics, styrene-modified polyester varnishes and alkyds. The important factor in using these types of varnishes is that the varnish must be reactive at low temperatures in order that the baking or curing operations will not injure the coil insulation.

A combination varnish and oil impregnation has the above stated advantage of supplying a higher short-circuit strength, but usually has been found to impart the disadvantage of much longer heating or baking cycles at elevated temperatures. High temperature baking may materially affect insulation life. In addition, a coil having a completely penetrating varnish impregnation has been found to have inherently weaker surge strength than an oil impregnated coil.

It is, therefore, a primary object of this invention to provide a new method for the impregnation of transformer coil and core assemblies embodying both the advantage of maximum short-circuit strength and surge strength by combining an oil impregnation with an effective superficial layer of varnish impregnant.

Another object of this invention is to provide a new method of impregnation for transformer coil and core assemblies utilizing a combined oil and varnish irnpregnation, said oil impregnation penetrating the in- 2,707,155 Patented Apr. 26, 1955 terior layers of coil windings and the varnish impregnation providing an elfective superficial layer penetrating the outer layers of coil windings, thus providing a transformer having the excellent short-circuit characteristics of a varnish impregnation and, in addition to maximum surge strength provided by oil impregnation.

A further object of this invention is to provide a transformer impregnating method that will impart a shelllike circumfused layer of heat-reactive varnish requiring a relatively lower baking or curing temperature, preventing deleterious effects upon the transformer insulation, said method also including an oil impregnation which penetrates the innermost layers of the transformer coil, the combined impregnation providing the inherent advantages of high surge strength and maximum shortcircuit strength.

Still another object of this invention is to provide as a new article of manufacture an electrical induction winding assembly having a circumfused layer of heat reactive varnish in addition to having its innermost winding layers impregnated with dielectric oil.

For a clear understanding of the nature and objects of the invention, reference should be had to the following detailed description and drawing in which;

Fig. 1 is a pictorial view depicting the first step in the novel impregnation process, in which previously fabricated core and coil assemblies are placed in an oven at elevated temperatures for a predetermined period as a means of drying insulation prior to impregnation.

Fig. 2 is a pictorial view depicting the varnish-dip stage following the drying step described in Fig. 1.

Fig. 3 is a pictorial view depicting the removal of the core and coil assemblies from baking or curing ovens in which the assemblies have been placed subsequent to the varnish-dip stage.

Fig. 4 is a pictorial view depicting the oil-impregnation stage which is accomplished by simply immersing a coil, or coil and core assembly, in a previously oilfilled transformer tank.

Fig. 5 is a pictorial view depicting the final stage of the novel transformer impregnation process, and illustrates the assembly operation relative to fastening of the various terminal connections performed prior to sealing of the tank cover and testing of the transformer unit.

Fig. 6 is a diagrammatic view of the improved transformer coil and core assembly manufactured under the novel impregnation process.

Referring now to Fig. l of the drawing, there is illustrated the first stage of the novel process performed on a previously fabricated core and coil assembly 1. it will be apparent that the coils 2 may be individually impregnated under this method prior to assembly on the core 3. However, for illustrative purposes, there is de picted a completed coil and core assembly 1, manufactured under the practice disclosed in Patent No. 2,305,999, granted to Alwin G. Steinmayer and William E. Krueger on December 22, 1942, and assigned to the same assiguee as the present invention. As disclosed in this patent, a coil 2 is wound directly upon a wound magnetic ribbon core 3. This may be illustrated most clearly with reference to Fig. 6.

This first stage is an insulation drying stage. The assembly 1 is placed in an oven 4 which is preferably provided with a means for obtaining air circulation. The

dry, circulating air of the oven will then completely dry out insulation maintaining minimum percentages in power factor. After the units have obtained maximum dryness, they are next brought to the varnish-dip stage depicted in Fig. 2.

At the varnish-dip station, the assemblies are preferably immersed in varnish to a depth sufficient to completely cover the coils 2. This may be conveniently performed in an open tank 5. As it is desirable to provide only a superficial layer of varnish, the coils are immersed for a comparatively short period. This period may be from seconds to 4 minutes, depending upon the size of the transformer. The immersion time is preferably predetermined to permit the outside two or three layers of insulating paper to be completely impregnated with the sides of the coil insulation impregnated approximately to 1" deep.

As has been previously stated, the preferred varnish is a heat reactive, oil-modified phenolic. However, it will be apparentthat other types of varnishes will'be of equal value, such as styrene-modified polyester varnishes and alkyds. The important consideration is that the varnish must be reactive at low temperatures in 'order that the baking or curing oven temperature may be kept sufficiently low to prevent injury to the insulation.

'After the units have been removed from the varnishthey-may be suspended above the tank 5 as shown in Fig. 2 to drain excess varnish from the unit. Varnish inspection operations may be performed during this draining period.

The assemblies are next placed in baking or curing ovens 6 as illustrated in Fig. 3. It will be apparent that the ovens shown may be the same ovens 4 used for drying insulation as depicted in Fig. 1. However, the temperature of the oven is preferably retained .at a level high 'enough'to cure the heat-reactive varnish, but at the same time sufiiciently low to prevent injury to insulation. During this curing operation, the varnish volatilesare driven from the impregnated layers, leaving void spaces in the relatively thin varnish layer. vThese void spaces are utilized as will hereinafter be described. Long curing cycles are not necessary as with ordinary varnish or resinous impregnations because the varnish .is not permitted to penetrate the innermost coil layers.

The next station in the novel processis depicted in Fig. 4, and is merely the operation ofplacing the coil and core assemblies 1 in a previously oil-filled transformer tank 7. The tank 7 contains a sutficient amount of oil 'to permit penetration through the void passages in the superficial varnish layer to the innermost coil winding layers. Since only a superficial layer of varnish is used, the oil readily impregnates the interior portion of the coil, as may be particularly shown in Fig. 6, wherein the superficial layer of varnish is indicated by reference character 8 with the oil impregnation penetrating through the varnish layer to the innermost coil windings 9.

It will be obvious that the oil impregnation stage may be accomplished by well-known vacuum impregnating devices (not shown). These devices are most effective when corona disturbances may be a major factor. Complete removal of entrained air is often desirable for minimizing the effects of corona and its accompanying disturbing influences.

a dry stage in order to reduce the power factor, which is directly dependent upon the degree of moisture contained in the transformer insulation. At the finishing station the transformer cover (not shown) is placed on the unit, and the unit is tested according to established standards to determine its surge strength,.power factor, etc. After passing inspection the transformers are crated and pre pared for shipment (not shown).

The combination of the protective vise-like envelope of varnish and deeply penetrating oil impregnatiomas illustrated in cross-section of Fig. 6, contribute to provide a transformer assembly which has a surge strength at least equal to a unit treated with oil alone.

The varnish baking or curing temperature is held at a relatively low value in order that there will be no deleterious effects upon the insulation. The outside insulation layer is protected from 'heat damage during baking or curing by the wet varnish layer until the varnish has hardened. In addition, the internalinsulation of the coil is protected by the varnished external insulation of the coil, which acts a as heat shield until the assembly slowly warms to predetermined curing temperature.

It will be apparent that the expression transformer, as used throughout this specification, is to be considered in its broadest sense. That is, it is to include any electrical induction apparatus utilizing impregnated winding assemblies.

It will be apparent that there has been provided a novel transformer assembly and method of impregnating said assembly which is comparatively inexpensive to manufacture, and which provides a transformer with maximum short-circuit and surge strengths, minimum Weight of varnish, and reduced power factor. In addition, the

method is basically simple, in that it does not require elaborate air evacuation equipment including covered dipping tanks, special air conditioning units, and other equipment requiring great expense of installation and maintenance.

Having now described my invention, what I claim as new, and desire to secure by Letters Patent is:

l. The process of impregnating a porous conductive winding coil for an electrical induction apparatus which consists in evaporating moisture from said coil; immersing said coil in a heat-reactive varnish mixture containing volatile varnish solvents, said immersion providing only a superficial varnish impregnation layer; removing said coil from said mixture; curing said varnished coil to evaporate said volatile solvents from said layer to leave void spaces therein and to provide a non-brittle porous texture to said varnish layer; and impregnating said coil in dielectric oil, said oil permeating the interior portion of said coil through the said void spaces of the varnish layer.

2. The process of impregnating a porous conductive winding coil for an electrical induction apparatus which consists in evaporating moisture from said coil; immersing said coil for a predetermined period in a heat-reactive varnish mixture containing volatile varnish solvents to provide only a superficial varnish impregnation layer; curing said varnished coil to evaporate said volatile solvents from said layer to leave void spaces therein and to provide a non-brittle porous texture to said varnish layer; and impregnating said coil in dielectric oil, said oil pertreating the interior portion of said coil through the ing said coil in a heat-reactive oil-modified phenolic varnish mixture containing volatile varnish solvents, said immersion providing only a superficial varnish impregnation layer; removing said coil from said mixture; curing said varnished coil to evaporate said volatile solvents from said layer to leave void spaces therein and provide a non-brittle .porous texture to said varnish layer; and impregnating said coil in dielectric oil, said oil permeating the interior portion of said coil through the said void spaces of the varnish layer.

4. The process of impregnating 'a porous conductive winding coil for an electrical induction apparatus which consists in evaporating moisture from said coil; immersing said coil in a heat-reactive styrene-modified polyester varn1sh mixture containing volatile varnish solvents, said immersion providing only a superficial varnish impregnation layer; removing said coil from said mixture; curing said varnished coil to evaporate said volatile solvents from said layer to leave void spaces therein and provide a non-brittle porous texture to said varnish layer, and impregnating said coil in dielectric oil, said oil permeating the interior portion of said coil through the said void spaces of the varnish layer.

5. The process of impregnating a porous conductive winding coil for an electrical induction apparatus which OI1SlSi S m evaporating moisture from said coil; immersing said coil in a heat-reactive alkyd-type varnishmixture containing volatile varnish solvents, said immersion providing only a superficial varnish impregnation layer; evaporating said volatile solvents from said layer to leave void spaces therein and provide a non-brittle porous texture to said varnish layer; and impregnating said coil in dielectric oil, said oil permeating the interior portion of eaid coil through the said void spaces of the varnish ayer.

6. porous conductive winding coil for an electrical nductive apparatus having its outer coil windings only impregnated with a heat-hardened non-brittle porous varnish mixture and a dielectric oil disseminated throughout the interstices'of said coil and of said porous varnish mixture.

7. porous conductive winding coil for an'electrical llldllCtlVe apparatus having its outer'coil windings only impregnated with a'heat-hardened non-brittle porous oilmodified phenolic varnish mixture and a dielectric oil disseminated throughout the interstices of said coil and of said porous varnish mixture.

8. porous conductive winding coil for an'electrical inductive apparatus having its outer coil windings only impregnated with a heat-hardened non-brittle porous styrene-modified varnish mixture and a dielectric oil dis seminated throughout the interstices of said coil and of said porous varnish mixture.

9. A porous conductive winding coil for an electrical inductive apparatus having its outer coil windings only impregnated with a heat-hardened non-brittle porous alkyd-type varnish mixture and a dielectric oil disseminated throughout the interstices of said coil and of said porous varnish mixture.

References Cited in the file of this patent UNITED STATES PATENTS Work June 17, 1941 Flynn Mar. 15, 1949 Foster Oct. 11, 1949 Robinson Oct. 24, 1950 

1. THE PROCESS FOR IMPREGNATING A POROUS CONDUCTIVE WINDING COIL FOR AN ELECTRICAL INDUCTION APPARATUS WHICH CONSISTS IN EVAPORATING MOISTURE FROM SAID COIL; IMMERSING SAID COIL IN A HEAT-REACTIVE VARNISH MIXTURE CONTAINING COLATILE VARNISH SOLVENTS, SAID IMMERSION PROVIDING ONLY A SUPERFICIAL VARNISH IMPREGNATION LAYER; REMOVING SAID COIL FROM SAID MIXTURE; CURING SAID VARNISHED COIL TO EVAPORATE SAID VOLATILE SOLVENTS FROM SAID LAYER TO LEAVE VOID SPACES THEREIN AND TO PROVIDE A NON-BRITTLE POROUS TEXTURE TO SAID VARNISH LAYER; AND IMPREGNATING SAID COIL IN DIELECTRIC OIL, SAID OIL PERMEATING THE INTERIOR PORTION OF SAID COIL THROUGH THE SAID VOID SPACES OF THE VARNISH LAYER. 